Alternating 1-0 Triangle in Python

Beginner
⏱️ 6 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Modulo Operator
Alternating 1-0 triangle output (1, 01, 101, 0101, 10101) in Python programming

What You’ll Learn

How to print an alternating 1-0 triangle in Python using nested loops and the modulo operator (%). Each row alternates digits based on parity.

This pattern is a great way to practice how parity and modulo can drive pattern output.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
1
01
101
0101
10101
1

Complete Python Program

The inner loop prints j % 2, which alternates between 0 and 1 as j increases.

Python
rows = 5

for i in range(1, rows + 1):
    for j in range(i, 2 * i):
        print(j % 2, end="")
    print()

🧠 How It Works

1

Pick the number of rows

rows = 5 controls the triangle height.

Setup
2

Outer loop (row index)

for i in range(1, rows + 1) sets how many digits each row will have.

Row control
3

Inner loop prints consecutive j values

for j in range(i, 2 * i) generates i consecutive numbers, which makes each row longer than the previous.

Loop range
4

Modulo gives 0/1 alternation

j % 2 is 0 for even numbers and 1 for odd numbers, so the output alternates as j increments.

Modulo
=

Alternating 1-0 triangle

Total digits printed are 1+2+…+n = n(n+1)/2, so time complexity is O(n²) for n rows.

2

Variation — User Input Version

Let the user decide the number of rows at runtime using input():

Python
rows = int(input("Enter the number of rows: "))

for i in range(1, rows + 1):
    for j in range(i, 2 * i):
        print(j % 2, end="")
    print()

💡 Tips for Enhancement

Try These

  • Validate input (reject rows < 1) before printing the pattern
  • Swap j % 2 to (j + 1) % 2 to flip starting digits
  • Print spaces between digits to make the triangle more readable
  • Try the same idea with j % 3 to cycle digits 0-1-2
  • Build the row as a string and print once for performance on large rows

Avoid

  • Forgetting print() between rows
  • Mixing up the inner loop range (it must produce i digits)
  • Assuming user input is always valid
  • Using print() inside large loops without considering output cost

Key Takeaways

1

The inner loop prints j % 2, which alternates 0 and 1.

2

The range range(i, 2*i) prints exactly i digits per row.

3

Modulo-based patterns are a simple way to generate repeating sequences.

4

Total digits printed follow the triangular number count: n(n+1)/2.

❓ Frequently Asked Questions

Because the first value in that row’s range is even, and even numbers give j % 2 = 0.
Print (j + 1) % 2 instead of j % 2 to flip 0 and 1.
Yes. Change end="" to end=" ". If you want no trailing space, build the row and join it.
O(n²) for n rows: total prints are 1+2+…+n = n(n+1)/2.

Explore More Python Number Patterns!

Keep going to discover more number pattern programs and strengthen your loop skills.

All Number Patterns →
Did you know?

Modulo arithmetic is widely used in programming—from alternating patterns and cyclic buffers to hashing. In pattern printing, it’s one of the simplest ways to repeat and alternate output.

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

12 people found this page helpful

Python Number Pattern 14 Python Number Pattern 16